The invention relates generally to the field of telecommunications, and more particularly to systems and methods for improving the performance of data transmissions in wireless telecommunications systems.
In a typical wireless voice/data communications system, a base station is associated with an area of coverage. This area is referred to as a sector. Mobile stations which are within the sector can transmit data to the base station and receive data from the base station. In the context of data communications in particular, base stations may be referred to as access networks, and mobile stations may be referred to as access terminals. Access terminals can communicate simultaneously with more than one access network and, as an access terminal moves, the set of access networks with which it communicates may change.
The parameters for communications between a particular access network and a particular access terminal are based in part upon their relative positions and the quality and strength of the signals that are respectively transmitted and received by them. For example, as the access terminal moves farther away from the access network, the strength of the signal received from the access network by the access terminal will decrease. Accordingly, the error rate of the received data will increase. The access network will therefore typically compensate for the increased distance by reducing the rate at which it transmits data to the access terminal. This allows the access terminal to receive and decode the access network's signal with fewer errors. When the access terminal moves closer to the access network, the signal strength increases, so a higher data rate can be used to transmit data to the access terminal.
Similarly, as the access terminal moves farther away from the access network, the strength of the signal received from the access terminal by the access network will decrease, thereby potentially resulting in a higher error rate. Like the access network, the access terminal will typically also compensate for the increased distance by decreasing its data rate to allow the access network to receive the signal with fewer errors. The access terminal may also increase its power output to reduce the error rate if requested by the access network. Again, when the access terminal moves closer to the access network, the stronger signal will support a higher data rate.
In one system, the access terminal is responsible for determining the rate at which data will be transmitted from the access terminal to the access network. This rate is determined based upon a number of factors. The primary factors are the absolute maximum rate at which the access terminal and access network can communicate, the maximum rate based upon the allowable power output of the access terminal, the maximum rate justified by the amount of data which the access terminal has in queue, and the maximum rate allowable based upon ramp-up constraints. In this system, each of these rates presents a hard limit that cannot be exceeded by the selected data rate. In other words, the selected data rate is no higher than the minimum of these four rates.
The first two of these rates (the absolute and power-limited maximum rates) result from physical constraints of the system and are outside the control of the access terminal. The third and fourth rates (the data-justified and ramp-up-limited rates) are variable and are dynamically determined based upon the specific prevailing conditions at the access terminal.
The data-justified rate is essentially the maximum rate that can be justified by the amount of data that is queued for transmission by the access terminal. For example, if the access terminal has 1000 bits in its transmit queue, then a data rate of 38.4 kbps (1024 bits/frame) is justified, but a rate of 76.8 (2048 bits/frame) is not justified. If there is no data in the access terminal's transmission queue, then no transmission rate at all is justified.
The ramp-up-limited rate is the maximum rate that is allowed, considering the fact that a rapid ramp-up will suddenly increase the interference perceived by other access terminals and will degrade their performance. If the ramp-up of each access terminal is limited, then the level of interference which it causes will change more slowly and the other access terminals can more easily adjust their operating data rates and transmit powers to adapt to the increased interference. It should be noted that the ramp-up-limited rate is also computed to control the ramp-down of data rates. The overall effect is to minimize wide and/or rapid fluctuations in data rates and to thereby stabilize the overall operation of the access network and access terminals in the system.
While the change in the ramp-up-limited rate is controlled (in regard to both increasing and decreasing data rates), the data-justified rate is not. If the access terminal suddenly has enough data to justify a very high rate, the data-justified rate will suddenly increase. If the access terminal runs out of data, the data-justified rate will suddenly drop to zero. Sudden increases in the data-justified rate typically are not problematic because the ramp-up-limited rate is controlled. Since the minimum of the four rates noted above sets a maximum for the selected data rate, the ramp-up-limited rate will control in this situation. Sudden decreases in the data-justified rate will, however, cause the actual data rate to drop since the data-justified rate is lower than the other rates and will therefore control (keeping in mind that the data rate selected for transmission of data over the next frame is the minimum of the four rates).
In prior art systems, if an access terminal has no data to transmit, no data is transmitted. This is certainly intuitive, and conventional wisdom dictates that useful bandwidth should not be wasted by transmitting useless data. One of the problems that results from allowing the data rate to drop precipitously (to zero, for example) is that it takes some amount of time for the data rate to ramp back up, as explained above. Delays in the transmission of some data may result from the drop and subsequent ramping up of the data rate. This delay is particularly likely in the case of data that is bursty or has discrete arrival processes. One such type of data is real-time video which may comprise 500-1000 byte packets that arrive at the transmit queue at discrete intervals of 60-70 milliseconds. Real-time video is also a notable example of the types of data for which transmission delays are particularly noticeable and therefore unacceptable.
It should also be noted that, while the ramp-up-limited rate is designed to prevent an access terminal from increasing its data rate in a manner that produces too much interference for other access terminals, there are instances in which the additional interference is not too disruptive. If there are few access terminals active in the sector, it may be acceptable for a particular access terminal to increase its data rate more rapidly than allowed by the ramp-up-limited rate. In such situations, the limit imposed by the ramp-up-limited rate may reduce the overall performance of the system.